By a known process, audio frequency pilot signals produced by a central station and transmitted to a plurality of locally spaced transmitting stations are synchronized in phase and transmitted to a common network. The pilot signals are controlled by a control signal, carried by a carrier signal derived as a subharmonic of the line voltage frequency (typically 50 or 60 Hz) and fed to the transmitting stations. The received control signal is synchronized at the transmitting stations by comparison to line voltage frequency. Such a process has been described in German Offenlegungsschrift No. 1,951,444, dated Jan. 28, 1971.
The aforementioned process is characterized by good synchronization and adherence to the phase of the audio-frequencies, a high degree of safety against breakdowns by line voltage harmonics and a low required band width for the transmission between the central station and the individual transmitting stations. However, the process is disadvantageous in that the apparatus necessary for transmitting the control signal in accordance with a carrier frequency process involves considerable expense.
In another known audio-frequency-heterodyne process, described in Nachrichtentechnische Fachberichte 31 (May 1966) pp 51 to 53, a subharmonic of the central control frequency is formed on the transmitting side, receiving a heterodyne process with two frequencies and single-side band transmission of the control frequency on a channel having a band width of 120 Hz. This subharmonic is then converted into two frequencies by multiplication with different factors. These frequencies are then mixed additively after scanning with the scanning frequency and are fed by a channel filter into the transmitting channel. The difference frequency of the two previously mentioned frequencies is formed in the receiver which is identical with the subharmonic and from this is then formed by multiplication the central control frequency scanned with the central controlled program. However, this process like the other known process involves expensive modes of transmitting and receiving.
Accordingly it is a primary object of the present invention to provide an inexpensively realized audio-frequency-heterodyne means and method.
Another object of the present invention is to provide audio-frequency-heterodyne means and method which are not dependant on line voltage frequency.
Yet another object of the present invention is to provide an audio-frequency-heterodyne system which may be fabricated from simple, commercially available integrated circuits.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the appended drawings.